I think that there are a couple of elements to this question that have to be addressed. The first would be that I think the concept of "African- Americans" came into being through the issue of slavery. Africans who were brought to America as slaves became "African- Americans." The fact that Africans were used as slaves came about through the colonization and spread of exploration from the fifteenth centuries. Linking Africa and the New World through the West Indies formed a route of triangular trade options. Goods such as fish and lumber were traded to the West Indies in exchange for sugar and molasses, used to make rum and other products, which were traded to West Africa for slaves. The more money that was made, the more slavery became a part of the reality for many Africans. This triangular trade and its geography became a primary reason as to why Africa became a resource for slavery. Other nations and people were not able to play the role in this triangle.
Both good answers. There were, however, European slaves in North America, although a small number. Many were persons kidnapped in the British Isles in the aftermath of the Jacobite rebellions, and sold in the New World both in the Caribbean and the mainland British colonies. Some were sold and then treated as indentured servants, but some were kept in slavery. The earliest African slaves brought to the British colonies were treated as indentured servants and set free after seven years with pay and land, until 1644 when the Massachusetts legislature passed the "Body of Liberties" bill legalising chattel slavery in British possessions on the mainland. The Spanish of course enslaved Indians, Africans and a great many captured British, Dutch and French sailors indiscriminately.
So while we tend today to concentrate on slavery in the New World as the enslavement of Africans, in reality blacks, whites and Native Americans were all enslaved at times. Entire populations, such as the native Jamaicans, were exterminated by the Spanish. And while Central Africa was nearly denuded of population by the slave trade almost all of those slaves were sold to Arabic countries or India, and the vast majority in the 1870s and '80s, after the American "Civil War" was over.
It's also important to note that American Indians were enslaved across the country, although at much fewer numbers than Africans. The Spanish missionaries in California had the largest population of enslaved natives, although the encomienda system kept many enslaved in Florida, & even the Puritans "sold the survivors of the Pequot War into slavery in Bermuda in 1637." That quote is from James Loewen's Lies My Teacher Told Me. The chapter entitled "Red Eyes" has abundant information on the period in history, including covering that native tribes began to enslave each other, to sell to the Europeans for guns and other goods. In addition, settlers began transporting native slaves (who were able to escape while in their native country) to the West Indies in exchange for African slaves. So the native slave trade actually drove and influenced growth of the African slave trade as well. Along with rampant disease, the slave trade was one of the leading factors in decimating the native population soon after European settlement.
Why people are forced or not forced into slavery is purely a matter of the ability of the people using the slaves to continue to keep the slaves in bondage. It was easier to keep in bondage people who captured in Africa and transported to a place far away from their own people, then say native Americans who had support of their own people closer by.
It is not as if there were no attempt to enslave people from other communities also. But such attempts could not be sustained for long. In addition to slavery, there are also example of other practices, as bad as slavery, enforced upon people of own country also, whenever it was possible and considered to be in the interest of their masters. For example the condition of labour force in textile mills in Britain during eighteenth and nineteenth century was perhaps as bad as condition of slaves in America. Perhaps the rate of mortality of these textile workers was much more than the African slaves working in USA.
Based on my research, there were Europeans that were used as slaves, mostly the Irish; many captured during the war with England. England wanted to get rid of them and therefore, sent them to America as slaves. However, they indentured slaves and was allowed to become free citizens after 7 years of service. As for the Indians, they too were forced into slavery, however, they were not physically able to withstand the heat or harsh labor and treatment - many died as a result. The African slaves on the other hand were able to handle the heat and conditions they were forced to work in.